HR
X
-C-HB-3
V
IA
L
ITE
HD
C-B
AND
L
INK
H
ANDBOOK
23
3
System integration
3.1
Link loss budget calculations
The link gain (transmitter RF input level to receiver RF output level) depends on the following factors:
Optical loss (due to connector insertion loss and optical fibre loss).
Transmitter gain setting.
Receiver gain setting.
The actual link gain can be determined as follows:
Link gain = Transmitter Gain + Receiver Gain
– (2 x optical loss) [dB]
(Where optical loss = connector insertion fibre losses)
3.2
Optical loss versus gain
The additional electrical insertion loss in dB resulting from optical losses is equal to 2 times that of the
optical loss in dB. This is due to the physics of the optical-to-electrical conversion process in the
receiver. For example, a 1dB increase in optical insertion loss will result in a 2dB decrease in RF
signal at the output of the optical receiver.
For single-mode fibre (e.g. SMF28), the optical loss at the 1310nm operating wavelength of the
ViaLiteHD
link is 0.4dB/km. For 1550nm operating wavelength, the optical loss is 0.2dB/km.
This can increase if the fibre is under excessive tension, compression or is bent into a small radius.
For clean, undamaged single-mode connectors, optical insertion loss is typically 0.2dB per interface.
Note: The losses at the optical connections of the transmitter and receiver are allowed for during
manufacture of the module, and may be ignored during link gain calculations.
For short links (<250m) containing no additional optical connectors, and in which the fibre is not
subject to any strain, the optical path loss can be ignored.
3.3
Optical loss versus noise figure
As the optical loss increases there will be a corresponding increase in noise, the chart below shows
the approximate relationship of optical loss to noise figure increase for a standard L-Band HTS link.
Below are graphs that shows the change in noise figure of some popular link types. For links with
high power transmitters see section 0.
Note: If you operate the
ViaLiteHD
modules in RLL AGC mode it is possible to mask optical loss
variations, but this is not always desirable.
0
5
10
15
20
25
30
35
0
2
4
6
8
10
12
14
16
18
20
N
o
ise
F
igu
re
In
cr
e
as
e
(
d
B
)
Optical link loss (dB)
Noise Figure increase vs Optical link loss
5km @1310nm
10km @1550nm
15km @1310nm
30km @1550nm
25km @1310nm
50km @1550nm
35km @1310nm
70km @1550nm
